New data obtained by studying the composition of diatom complexes of bottom sediments fromlakeTridzatka, located in the South of theMurmanskregion, are presented. According to the composition of diatom complexes, precipitation accumulation time is divided into four diatom zones corresponding to the stages of its development. At the initial stage of development of the lake sedimentation took place in conditions of glacial lake, which was then replaced by sea. On the basis of lithological data in the context at a depth of 708-710 mthere is a clear and sharp contact indicating a rapid change of sedimentation. According to radiocarbon analysis, the connection of the reservoir with the sea ceased in the middle Holocene. At the same time, it established conditions favorable for the development of freshwater diatom flora.